Some Contemporary Advances in Physics -V 



By KARL K. DARROW 



Electricity in Solids 



IN considering such topics as the flow of elcctriiil\' tlirouuli solids 

 and tlu' oiittlow of cii-ctricity across their boundaiies, we have to 

 forego the assistance of the great system of laws, models, and word- 

 pictures which constitutes the contemporary theory of the structure 

 of the atom. This imposing and truly powerful theory, which now- 

 adays seems to bulk larger than all of the rest of physics, is after all 

 limited to certain restricted fields; it deals successfully with par- 

 ticular properties of isolated atoms, and also with certain qualities 

 of atoms which seem to be localized in their inner regions; but it avails 

 little or nothing in the study of the behavior of liquids and solids. 

 Much of the present-day thcor>' of electrical conduction in solids is 

 based only on the very simplest assumptions as to the nature of the 

 atoms of which they are built, some would even remain valid under 

 the old-fashioned ideas of continuous electrical fluids; and profoundly 

 as we may believe that solids are built of atoms resembling Bohr's 

 famous model, it is highly doubtful whether that model has ever 

 helped to interpret a single one of the phenomena of conduction or 

 done more than to provide a new language for old ideas. 



We have first to make the distinction between the substances in 

 which atoms migrate along the path of the flowing current and ap- 

 parently carr\- the moving charge, and the substances in which the 

 atoms stand still while the current flows past them. It is universally 

 conceded that elements, and likewise the alloys of metals and a num- 

 ber of solid compounds, belong to the latter class; whatever it is that 

 carries the current flows through and past the substance, leaving it 

 at the end as it was at the beginning. Weber said in 1858, "In the 

 metals there are electrically-charged particles as well as atoms; some 

 of the former are freely rpobile and others vibrate about the atoms; 

 they are the cause of the confluction of electricity and of heat, and of 

 magnetic phenomena as well." Considering that in Weber's day 

 electricity had never been observed apart from ponderable matter 

 and electrons were unknown, this is entitled to rank as a daring 

 anticipation. 



N'ext we have to distinguish between conduction by metals and 



conduction by non-metallic elements. Strictly we should begin by 



defining a "metal"; but this task had better be left to the chemists, 



as being really their affair; and they have found it no easy affair to 



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